I think ～よう is often used to express a feeling such as you are “thinking”, “considering”, “planing”, “deciding”, “guessing” or something.
You say “帰ろう”, “寝よう”, “買おう” to yourself when you are making a decision. “帰る” "寝る" “買う” may be inappropriate because it's nether a “judgement” (because you always know exactly what you will do) nor a “statement” (because nobody needs this information.) You just directly express this kind of feeling using “～よう” no matter there is a listener or not. I think expressing one's emotions is something you often do without any reasons.
～ましょう might be used in certain situations, In novels, some people (e.g. お嬢様) just use the polite form exclusively. In narratives, the speaker is aware of the existence of the audiences, so he uses the polite form. But the ～ましょう in this case is not the speaker's feeling, but the the character's. Maybe the author of the children's book deliberately chose the polite form throughout the book, did he?
When you are speaking to a listener, you often change ～よう to ～ようと思います and ～よう carries some additional meanings. It seems that they think ～よう sounds too direct. They think 気持ちを抑えめに表現する＝丁寧に表現する.
I think これを買いましょう often implies the speaker decided to buy, tell the listener and ask for confirmation.